Automatic determination of the contamination of aqueous cleaning solutions with carbonaceous compounds

ABSTRACT

A method of automatically determining the content of inorganic carbon and/or total organic carbon in an aqueous purifying solution wherein, in program-controlled manner, (a) a sample of a predetermined volume is taken from the aqueous purifying solution; (b) if desired, the sample is freed of solids and/or homogenized; (c) if desired, the sample is diluted with water in a ratio which has been preset or is determined as a result of a preliminary analysis; (d) the inorganic carbon and/or total organic carbon is analysed using known methods; and (e) the result of the analysis is transmitted to a remote location and output and/or stored on a data carrier and/or used as the basis of further calculations. Program-controlled or automatic, externally-initiated checking of the measuring device is provided. Bath treatment measures may be implemented in program-controlled manner or in response to external triggering.

FIELD OF THE INVENTION

This invention relates to a method of automatically monitoring andcontrolling purifying baths wherein the content of inorganic carbon (IC)or total organic carbon (TOC) or the sum thereof (total carbon TC) inthe aqueous purifying solution is determined as measurement and controlparameters. The method is conceived in particular for commercialpurifying baths in the metal-processing industry and, for example, inautomobile construction. It permits, for example, automatic monitoringof the loading of the purifying bath, in particular with fats and oils,characterised by the parameter TOC, and if necessary the supplementationof the purifying bath or the initiation of other bath treatment measuresautomatically or in response to an external request. The method has beenconceived in particular such that the analysis results are transmittedto a location remote from the purifying bath. Furthermore, it ispossible to intervene in the automatic measurement process or initiatethe refilling or other bath treatment measures from a: location remotefrom the purifying bath. The “location remote from the purifying bath”may be situated in a superordinate process control system, in a controlcenter of the plant in which the purifying bath is situated, or also ata location outside the plant.

BACKGROUND OF THE INVENTION

The purification of metal components prior to further processing thereofconstitutes a routine task in the metal-processing industry. The metalcomponents may be contaminated, for example, with temporary coatingswhich have dissolved away or leached out, pigment dirt, dust, metalrubbings, corrosion protection oils, jointing materials such as adhesiveresidues, cooling lubricants or deformation agents. Prior to the furtherprocessing, in particular prior to a corrosion protection treatment orcoating (for example phosphation, chromatization, anodization, reactionwith complex fluorides, organic coating etc) or prior to lacquering,these impurities must be removed by means of a suitable purifyingsolution. Spraying, dipping or combined processes may be used for thispurpose.

Industrial purifiers in the metal-processing industry are generallyalkaline (pH above 7, for example 9 to 12), but may also be acidic. Thebasic constituents of alkaline purifiers are alkalis (alkali hydroxides,-carbonates, -silicates, -phosphates, -borates) as well as non-ionicand/or anionic surfactants. As additional auxiliary components, thepurifiers frequently and/or anionic surfactants. As additional auxiliarycomponents, the purifiers frequently contain complex-forming agents(gluconates, polyphosphates, salts of amino acids such as ethylenediamine tetraacetate or nitrilotriacetate, salts of phosphonic acids,such as salts of hydroxyethane diphosphonic acid, phosphono-butanetricarboxylic acid or other phosphonic or phosphonocarboxylic acids),corrosion protection means, such as salts of carboxylic acids having 6to 12 carbon atoms, alkanolamines and foam inhibitors, such asalkoxylates of alcohols having closed end groups and 6 to 16 carbonatoms in the alkyl group. If the purifying baths contain no anionicsurfactants, cationic surfactants may also be used. Acidic purifierscontain acids, such as phosphoric acid or sulfuric acid, in place of thealkalis.

As non-ionic surfactants, the purifiers generally contain ethoxylates,propoxylates and/or ethoxylates/propoxylates of alcohols or alkylamineshaving 6 to 16 carbon atoms in the alkyl group, which may also haveclosed end groups. Alkylsulfates and alkylsulfonates are widely used asanionic surfactants. Alkylbenzene sulfonates are also encountered,although these are disadvantageous from the environmental standpoint. Inparticular, cationic alkylammonium compounds containing at least onealkyl group having 8 or more carbon atoms are suitable as cationicsurfactants.

SUMMARY OF THE INVENTION

As a result of the purifying process, the dirt constituents which havedissolved away from the surfaces accumulate in the purifying solution.Pigment dirt may lead to loading with inorganic carbon. Corrosionprotection oils, cooling lubricants or deformation agents, such asdrawing grease and/or organic coatings which have dissolved away orleached out or jointing materials, lead to the loading of the purifyingsolution with total organic carbon. As the majority of this totalorganic carbon is present in the form of mineral oils, mineral fats, oroils and fats of animal or vegetable origin, it is often referred to inabbreviated form as the “fat loading” of the purifying solution. Themajority of such oils and fats are present in emulsified form in thepurifying solution. Oils and fats of animal or vegetable origin may,however, be at least partially hydrolysed by an alkaline purifyingsolution. The hydrolysis products may then also occur in dissolved formin the purifying solution. Having too high a TOC loading of thepurifying solution, it is no longer guaranteed that the purifyingsolution will free the components to be purified of oils and fats to therequired extent. Alternatively, the danger exists that oils and fatswill be drawn back onto the purified components when these are removedfrom the purifying solution. Therefore. it is necessary to maintain thefat loading of the purifying solution below a critical maximum valuewhich may depend upon the further use of the purified components andupon the composition of the purifying solution. In the case of a highfat loading, it is possible to increase the surfactant content of thepurifying solution in order to increase the fat dissolving capacity ofthe purifying solution. Alternatively, bath treatment measures areinitiated with the goal of reducing the fat loading of the purifyingSolution. This is any case necessary at a predetermined maximum limit ofthe fat loading. In the simplest case, the purifying solution isentirely or partially discarded and replaced or supplemented with freshpurifying solution. However, on account of the waste water therebyproduced and due to the need for fresh water, it is endeavoured toseparate fats and oils from the purifying solution and to continue touse the purifying solution, optionally supplemented with activeingredients. Suitable devices for this purpose, such as separators ormembrane filtration apparatus, are known in tile art.

Previously, the purifying efficiency of a purifying solution wasconventionally assessed visually on the basis of the purificationresult. The plant operating personnel assess the purifying efficiencyand implement the required measures, such as bath supplementation orbath renewal. This currently customary method requires that operatingpersonnel remain in the vicinity of the purifying bath at the requiredmonitoring times. The shorter the desired monitoring interval, thegreater the demands upon the operating personnel for the visualassessment.

DETAILED DESCRIPTION OF THE INVENTION

By way of contrast, an object of the present invention is to implementand document the monitoring of purifying baths by determining thecontent of inorganic carbon and/or total organic carbon in an automatedmanner such that at least the results of the analysis are stored on adata carrier and/or output. Preferably, the measuring device used is tobe self-checking and self-calibrating and, in the case of a malfunction,is to transmit an alarm signal to a remote location. Furthermore, it ispreferably to be possible to check the functioning capability of themeasuring device and the measurement results from a remote location.Furthermore, it is to be possible to intervene in the measurementprocess and the bath treatment measures from a remote location. Byvirtue of the desired remote monitoring, the outlay in terms ofpersonnel for tile bath monitoring and bath control of tile purifyingbaths is to be reduced.

This object is achieved by a method for automatically determining thecontent of inorganic carbon and/or total organic carbon in an aqueouspurifying solution wherein, in a program controlled manner:

(a) a sample of a predetermined volume is taken from the aqueouspurifying solution;

(b) if desired, the sample is freed of solids and/or homogenized;

(c) if desired, the sample is diluted with water in a ratio which hasbeen preset or is determined as a result of a preliminary analysis;

(d) the inorganic carbon and/or total organic carbon is analysed usingknown methods; and

(e) the result of the analysis is transmitted to a remote location andoutput and/or stored on a data carrier and/or used as the basis offurther calculations.

The sample volume taken in (a) may be permanently programmed into thecontrol section of the measuring device to be used for the method.Preferably the size of the sample volume may be changed from a remotelocation. Additionally, the control program may be designed such that itmakes the sample volume to be used dependent upon the result of aprevious measurement and/or automatically dilutes the sample to adesired measurement range. For example, the sample volume may beselected to be the greater, the lower the fat loading of the purifyingbath. The accuracy of the analysis may thus be optimized.

Where the method according to the present invention refers to a “remotelocation”, this is to be understood as a location situated not in directcontact, or at least not in visual contact, with the purifying bath. Theremote location may for example be a central process control systemwhich, as part of a total process for the surface treatment of tilemetal components, monitors and controls the purifying bath as asubsidiary task. The “remote location” may also be an observationcontrol center from which the overall process is monitored andcontrolled and which is situated for example in a different room to thepurifying bath. The “remote location” may also, however, consist of alocation outside the plant in which the purifying bath is situated. Inthis way, it is possible for specialists present outside the plant inwhich the purifying bath is situated to check and control the purifyingbath. As a result, the presence of specialist personnel at the locationof the purifying bath is less frequently necessary.

Suitable data lines via which the analysis results and control commandsmay be transmitted are available in the prior art.

Between the taking of the sample and the actual measurement it may bedesirable to free the sample of solids in the optional step (b). This isunnecessary in the case of a purifying bath having only a low solidsloading. However, too high a solids content of the purifying bath maylead to obstruction of valves of the measuring device. Therefore it isadvisable to remove solids from the sample. This may take placeautomatically by filtration or also by the use of a cyclone orcentrifuge. It is advisable to homogenize the sample, for example byvigorous stirring. This leads to a uniform and fine distribution of theorganic impurities possibly present in the form of coarse oil or fatdroplets.

If necessary, in (c), the sample is diluted using water in a specifiedratio. This ratio may be fixed, but modifiable from a remote location.However, the dilution ratio may also be made dependent upon the resultof a previous analysis of the content of inorganic carbon and/or totalorganic carbon. This ensures that the carbon content of the samplesolution is in a range which permits optimal analysis using the selectedmethod.

In (d), the inorganic carbon and/or total organic carbon may beanalysed, for example by converting it into CO₂ and quantitativelydetermining the formed CO₂.

The conversion of the carbon into CO₂ by oxidation may be effected, forexample, by combustion at an elevated temperature in the gas phase. Theelevated temperature during the combustion is preferably greater thanabout 600° C., for example is about 680° C. Preferably, the combustionis carried out using air or oxygen gas in a reaction pipe aided by acatalyst. Suitable catalysts are, for example, noble metal oxides orother metal oxides, such as vanadates, vanadium oxides, chromium-,manganese- or iron oxides. Platinum or palladium deposited on aluminumoxide may also be used as catalyst. This process directly provides aCO₂-containing combustion gas whose CO₂ content may be determined asdescribed in the following.

As an alternative to combustion in the gas phase, the conversion of thecarbon into CO₂ may also be effected by means of wet chemistry. Here,the carbon of the sample is oxidized using a strong chemical oxidant,such as hydrogen peroxide or peroxodisulfate. If desired, thiswet-chemical oxidation reaction may be accelerated with the aid of acatalyst of the type referred to in the foregoing and/or withUV-radiation. In this case, it is preferable to expel the formed CO₂with a gas flow from the, if necessary acidified, sample forquantitative determination thereof. Carbon found in the form ofcarbonates or CO₂ may likewise be detected.

Irrespective of the method by which gaseous CO₂ has been generated, itmay be quantitatively determined in accordance with one of the followingmethods. When the quantity of the sample is known, the content ofinorganic carbon and/or total organic carbon in the purifying solutionmay be calculated therefrom. Alternatively, using a predeterminedconversion factor, the result of the analysis may be given in the formof fat loading per liter of purifying bath if inorganic carbon is notpresent or has previously been removed.

Different methods known in the prior art may be used to determine theCO₂ content of the obtained gas flow. For example, the gases may bepassed through an absorber solution and, for example, the increase inweight of the absorber solution may be measured. For example, an aqueoussolution of potassium hydroxide which absorbs CO₂ with the formation ofpotassium carbonate is suitable for this purpose. As an alternative todetermining the increase in weight, it is possible to determine thechange in the electrical conductivity of the absorption solution orresidual alkalinity thereof following the absorption of the CO₂.

The formed CO₂ may be absorbed by a suitable solid whose increase inweight is measured. For example, soda asbestos, is suitable for thispurpose. Naturally, it is necessary to replace both an absorber solutionand a solid absorber when they are exhausted and are no longer able tobind CO₂.

However, for an automatically operating process it is simpler toquantitatively determine the CO₂ content of the gas by measuring theinfrared absorption. The determination of the infrared absorption maytake place, for example, at a wavelength of 4.26 μm corresponding to awave number of 2349 cm⁻¹. Devices capable of performing the combustionof the sample and the measurement of the infrared absorption are knownin the prior art. The TOC system of the company Shimadzu is mentioned asan example.

For the photometric analysis of the CO₂ content of the combustion gasand the gas expelled from the sample, it is possible to use not onlydispersively operating infrared spectrometers, but also non-dispersivephotometers. These are also known as “NDIR devices”. Such a device isdescribed, for example, in DE-A-4405881.

In this analysis method, the proportion of carbon deriving fromdeliberately added active ingredients in the purifying solution is alsodetected. Surfactants, organic corrosion inhibitors and organiccomplex-forming agents are mentioned as examples. However, the contentthereof in the purifying solution is known within specific fluctuationlimits or may be separately determined. The proportion of total organiccarbon deriving from these active ingredients may thus be subtractedfrom the result of the analysis. The proportion deriving from theentered impurities is then obtained. In practice, it is not essential inthis case for the proportion of carbon present in the form of activeingredients to be taken into account in the carbon analysis. Rather, itis often sufficient to fix an upper limit of the carbon content of thepurifying solution which itself takes into account the active ingredientcontent. By means of the carbon analysis, it is then ascertained whetherthe carbon loading is below or above this maximum limit.

The proportion of total organic carbon present in the form of lipophilicsubstances may alternatively be determined such that the lipophilicsubstances are extracted into an organic solvent not miscible in allproportions with water. When the solvent has evaporated off, thelipophilic substances remain and may be gravimetrically analysed.Preferably, however, the infrared absorption of the lipophilicsubstances in the extract is photometrically analysed. Halogenatedhydrocarbons may be used in particular as organic solvent not misciblein all proportions with water. A preferred example is1,1,2-trichlorotrifluoroethane. This analysis method is based on DIN38409, part 17, but, in contrast to this method, the proportion oflipophilic substances in the sample is analysed not gravimetricallyfollowing the evaporation of the organic solvent, but photometrically inthe organic solvent. The quantitative analysis is preferably performedas in DIN 38409, part 18, by measuring the infrared absorption of thelipophilic substances in the extract at a characteristic vibrationalfrequency of the CH₂ group. Here, it is advisable for an organic solventwhich itself contains no CH₂ groups to be used for the extraction. Theinfrared absorption band at 3.42 μm (2924 cm⁻¹), for example, may beused for this photometric analysis. All the organic substances whichcontain CH₂ groups and may be extracted into the organic solvent are nowdetected. In part, these are also the surfactants in the purifyingsolution. If this surfactant constituent is not to be detected, it maybe separately determined by an alternative method and subtracted fromthe total result. If necessary, the distribution coefficient of thesurfactants between the purifying solution and the organic solvent notmiscible in all proportions with water must be previously determined. Inpractice, however, it may be sufficient to fix a maximum value of thepermissible loading of the purifying solution with lipophilic substanceswhich additionally takes into account the surfactant constituents. Ifthis maximum value is exceeded, bath treatment measures are to beinitiated.

As part of this method, it is advisable to calibrate infraredspectrometers using a known quantity of a lipophilic substance. Asolution of 400 to 500 mg methylpalmitate in 100 ml1,1,2-trichlorotrifluoroethane may be used, for example, as calibratingsolution. This calibrating solution is likewise used to monitor thefunctioning of the IR-photometer.

In this case, it is preferable to proceed by firstly adding a phosphoricacid magnesium sulfate solution to the sample of the purifying solution.This solution is prepared by dissolving 220 g crystalline magnesiumsulfate and 125 ml 85 wt. % phosphoric acid in deionised water andsupplementing this solution with deionised water to 1000 g. The samplesolution is mixed with about 20 ml of the phosphoric acid magnesiumsulfate solution. Then, 50 ml of the organic solvent not miscible in allproportions with water. preferably 1,1,2-trichlorotrifluoroethane, isadded. The aqueous and organic phases are mixed, a phase separation isperformed, and the organic phase is isolated. Preferably this organicphase is again washed with the phosphoric acid magnesium sulfatesolution, the phase separation is again performed and the organic phaseis drawn off. This is transferred into a measuring cuvette and theinfrared absorption is measured at a vibrational band of the CH₂ group.A suitable measuring cuvette consists, for example, of a quartz glasscuvette having a coating thickness of 1 mm. By comparison with thecalibration curve, which also contains the blind value of thephotometer, it is possible to determine the content of lipophilicsubstances in the sample on the basis of the infrared absorption.

Irrespective of the type of analysis method selected, the result of theanalysis is then output and/or stored on a data carrier (e). Here, thedata carrier may be situated at the analysis location or in a remotecomputer unit. “Output of the result of the analysis” is to beunderstood in that the result is either forwarded to a superordinateprocess control system or is displayed on a screen or printed out so asto be intelligible to a human. The location at which the result isdisplayed or output may correspond to the “remote location” indicatedabove. It is preferable for the results of the individual analyses to bestored on a data carrier at least for a predetermined time interval toenable them to be evaluated subsequently, for example in the form of aquality assurance check. However, the results of the carbon analysesneed not be directly output as such or stored on data carriers. Rather,they may also be used directly as the basis of further calculations, theresults of these further calculations being displayed or stored. Forexample, in place of the instantaneous carbon content, it is alsopossible to display the trend of the values and/or the relative changetherein. Alternatively, the instantaneous carbon contents may beconverted into “% of the maximum content”.

In the simplest case, the method according to the present inventionoperates such that (a) to (e) are repeated after a predetermined timeinterval. The predetermined time interval will depend upon therequirements of the operator of the purifying bath and may comprise anydesired time interval from a few minutes to several days. For qualityassurance, it is preferable for the predetermined time intervals torange, for example, between 5 minutes and 2 hours. For example, ameasurement may be performed every 15 minutes.

However, the method according to the present invention may also beimplemented in a such manner that (a) to (e) are repeated after timeintervals which are the shorter, the greater the difference between theresults of two consecutive analyses. The control system for the methodaccording to the present invention may thus itself decide whether thetime intervals between the individual analyses are to be reduced orincreased. Naturally, the instruction as to which time intervals are tobe selected in the case of which differences between consecutiveanalyses must be preset in the control system. It may also be providedthat the measurement intervals are coupled to the results of themeasurement of other contents. For example, the time intervals at whichthe inorganic carbon or total organic carbon in the purifying solutionis measured may be made dependent upon the results of a measurement ofthe surfactant content. Naturally, it is also possible externally topreset variable measurement intervals correlated, for example, with thematerial throughput through the purifying bath and/or with the knownaverage contamination of the material to be purified.

Furthermore, the method according to the present invention may beimplemented in such a manner that (a) to (e) are performed at a desiredtime in response to an external request. In this way, for example,immediate monitoring of the carbon content of the purifying bath may becarried out if quality problems are ascertained in following processsteps. The carbon measurement may thus take place in a time-controlledmanner (at fixed time intervals) or in an event-controlled manner (upontile ascertainment of changes or in response to external requests).

The present sampling and measuring system is preferably designed suchthat a central measuring unit may be supplied with samples fromdifferent purifying baths. In the relevant industrial sector, it iscustomary to purify metal components in a plurality of purifying bathsarranged in series. By means of sample lines leading to the individualpurifying baths, the carbon contents of the respective purifyingsolutions may be analysed consecutively using one single measuring unit.The measurement sequence of the individual baths may be presetexternally. Here, different measurement intervals may be provided forthe individual purifying baths so that, for example, one particularpurifying bath is checked more frequently than another. Furthermore, itmay be provided that the carbon content in a downstream purifying bathis not checked until the carbon content in an upstream purifying bathreaches a specified limit value.

In the implementation of the method according to the present invention,it may be desirable to detect both inorganic carbon and total organiccarbon (TOC). This is the case, for example, when the sample iscombusted for the analysis of the carbon content. Here, dissolved CO₂ orcarbon in the form of carbonates is additionally detected if CO₂ splitsoff from the carbonates at the selected combustion temperature. If inthis case the inorganic carbon is not to be additionally measured, itmay be removed in that the sample may be acidified and the formed CO₂ ispurged with a gas, such as air or nitrogen. This may be desirable if ina particular case only the “fat loading” of the purifying bath is to bedetermined. When the carbon content present in the form of lipophilicsubstances is determined in accordance with the above-describedextraction method, inorganic carbon is automatically not detected.

It is also possible for volatile organic compounds to be eliminated fromthe sample prior to the implementation of (d) by expulsion with a gas,such as air or nitrogen. For example, volatile solvents may beeliminated in this way prior to the carbon analysis.

Preferably the method according to the present invention is implementedin such manner that the measuring device used is self-monitoring and ifnecessary self-calibrating. For this purpose, it may be provided that,after a predetermined time interval or after a predetermined number ofanalyses or in response to an external request, the functioningcapability of the measuring device used is checked by controlmeasurements of one or more standard solutions. The check is carried outby measuring a standard solution containing known contents of inorganiccarbon and/or total organic carbon. This check is most realistic if astandard purifying solution whose composition is as close as possible tothat of the purifying solution to be checked is used as standardsolution. Standard solutions which do not constitute purifying solutionsmay likewise be used, however, for checking and/or calibration purposes.

If, during a control measurement of a standard solution, the measuringdevice determines a carbon content which differs from the nominalcontent by a minimum amount to be predetermined, the measuring deviceemits an alarm signal either locally or preferably at a remote location.The alarm signal may contain an intervention proposal selected by thecontrol program of the measuring device or by the superordinate processcontrol system.

In the method according to the present invention, it may also beprovided that the functioning capability of the measuring device used ischecked by a control measurement of one or more standard solutions ifthe results of two consecutive measurements differ by a predeterminedamount. In this way, it is possible to distinguish whether establisheddeviations in the carbon content of the purifying solution are real andnecessitate bath treatment measures or whether they have been simulatedby a fault in the measuring system.

Depending upon the result of the check on the measuring device used, theanalyses of the content of inorganic carbon and/or total organic carbonperformed between the current and the previous control measurement maybe provided with a status characteristic indicating the reliability ofthese analyses. If, for example, consecutive control measurements forchecking the measuring device used have shown that it is operatingcorrectly, the analyses of the carbon content may be provided with astatus characteristic “OK”. If the results of the control measurementsdiffer by a predetermined minimum amount, the intervening analyses maybe provided, for example, with the status characteristic “doubtful”.

It may additionally be provided that, depending upon the result of thecheck on the measuring device used, the automatic analysis of thecontent of inorganic carbon and/or total organic carbon is continuedand/or one or more of the following actions is performed: analysis ofestablished deviations, correction of the measuring device, terminationof the analysis of the carbon content, transmission of a status signalor an alarm signal to a superordinate process control system ormonitoring device, thus to a remote location. If desired, the measuringdevice may thus itself decide in accordance with preset criteria whetherit is sufficiently capable of functioning so as to allow the carbonanalyses to continue or whether deviations necessitating manualintervention are ascertained.

Preferably, the measuring system employed in the method according to thepresent invention is designed such that it automatically monitors thelevels and/or consumption of the standard and test solutions used, aswell as possible auxiliary solutions and upon the undershooting of apredetermined minimum level emits a warning signal. In this way it ispossible to prevent the measuring device from becoming incapable offunctioning due to a lack of the required solutions. The monitoring ofthe levels may take place in accordance with known methods. For example,the vessels containing the solutions may be placed on scales recordingthe particular weight of the solutions. Alternatively a float isinserted. Alternatively, a minimum level may be checked by means of aconductivity electrode submerged in the vessel containing tile solution.The warning signal to be emitted by the measuring device is preferablytransmitted to the remote location so that the appropriate measures maybe initiated from there. In general, in the method according to thepresent invention it is preferably provided that the results of theanalyses and/or of the control measurements and/or of the calibrationsand/or the status signals are transmitted to a remote locationcontinuously or at predetermined time intervals and/or upon request. Inthis way, the monitoring personnel, who are not required to be presentat the location of the purifying bath, are kept constantly informedabout the bath's instantaneous content of inorganic carbon and/or totalorganic carbon and thus about the current fat- and oil loading.Depending upon the result of the analyses and control measurements,necessary corrective measures may be adopted either automatically via aprocess control system or by manual intervention.

The simplest corrective measure consists in that, upon the overshootingof a predetermined maximum value of inorganic carbon and/or totalorganic carbon or in response to an external request, a device isactivated which dispenses one or more supplementary components (solutionor powder) into the purifying bath. This may take place, for example, inautomated fashion in that, depending upon the determined carbon content,a specified quantity of supplementary solution or supplementary powderis supplied to the purifying bath. Here it is possible to vary the sizeof the added portion itself or, in the case of fixed added portions, thetime interval between the individual additions. This may be effected,for example, via dosing pumps or also in weight-controlled fashion. Inthe method according to the present invention, it is thus provided, onthe one hand, that, in the case of specific deviations from the nominalvalue (in particular when the functioning capability of the measuringdevice has been ascertained by the control measurements), a specifiedquantity of supplementary component is additionally dosed into thepurifying bath. Furthermore, it may be provided that these bathsupplementing measures are performed when a predetermined minimum changein the carbon content has been established. Furthermore, however, thisadditional dosing may also take place in response to an externalrequest, for example from a remote location, independently of theinstantaneous carbon content. The additional dosing, for example ofsurfactants, increases the carbon content of the purifying solution.Upon the next analysis of the carbon content this must be taken intoaccount in an appropriate manner, which may take place automatically. Anaddition of surfactants increases the oil- and fat-bearing capacity ofthe purifying bath. Accordingly, it is necessary to Increase thetolerable maximum value of the carbon loading, the overshooting of whichinitiates the next bath treatment measure. This may be providedautomatically in the control program.

In place of an additional dosing of bath components, such assurfactants, or upon the overshooting of a predetermined maximum contentof inorganic carbon and/or total organic carbon, bath treatment measuresmay be initiated to reduce the content of inorganic carbon and/or totalorganic carbon in the purifying solution. Such bath treatment measureshave the goal in particular of reducing the fat and oil content of thepurifying solution. In the simplest example, this may take place in thatthe purifying solution is completely or partially discharged andreplaced by fresh purifying solution. It is more economical, however, toremove oils and fats from the purifying solution by measures known inthe prior art, such as separation by a separator or separation bymembrane filtration. As surfactants are also at least partiallydischarged in these processes, the purifying solution must besupplemented appropriately. The initiation of these measures may also bemade dependent not only upon the absolute carbon content of thepurifying solution but also upon a predetermined change in the carboncontent.

Naturally, the method according to the present invention requires thatthe appropriate device is available. This contains a control unit, forexample a computer control unit, which controls the measurement processin a time- and/or event-dependent manner. It must also comprise therequired vessels for solutions, pipelines, valves, dosing- and measuringdevices etc. for the control and measurement of the sample flows. Thematerials are to be adapted to the purpose of use, for example are toconsist of high-grade steel and/or plastics. The control electronicsunit of the measuring device is to possess an appropriate input-outputinterface to permit communication with a remote location.

The method according to the present invention, on the one hand, enablesthe carbon content of purifying baths to be checked on site andpredetermined corrective measures to be initiated without manualintervention. In this way, the process reliability is improved and aconstantly reliable purification result is obtained. Deviations from thenominal values may be detected at an early point in time and correctedautomatically or manually before the purification result is impaired. Onthe other hand, the measurement data are preferably transmitted to aremote location so that operating or supervisory personnel are keptconstantly informed about the state of the purifying bath, even whenthey are not present in the direct vicinity of the bath. The outlay interms of personnel for monitoring and controlling the purifying bath maythus be considerably reduced. The documentation of the data collected inthe method according to the present invention complies with therequirements of modern quality assurance. The consumption of chemicalsmay be documented and optimized.

What is claimed is:
 1. A method of maintaining a clear efficiency andadjusting the fat loading capacity of an aqueous purifying solution byautomatically determining the content of inorganic carbon and/or totalorganic carbon in the aqueous purifying solution wherein, in aprogram-controlled manner said method comprises the steps of, (a) takinga sample of a predetermined volume from the aqueous purifying solution,(b) if desired, freeing the sample of solids and/or homogenizing thesample, (c) if desired, diluting the sample with water in a ratio whichhas been preset or determined as a result of a preliminary analysis, (d)monitoring the total fat and oil loading of said solution by analyzingthe inorganic carbon and/or total organic carbon of said solution usingknown methods as an indicator of a fat and oil loading of said solution,(e) transmitting the result of the analysis of step (d) to a remotelocation and outputting said result and/or storing said result on a datacarrier and/or using said result as the basis of further calculations,and f1) activating a device and dosing one or more supplementarycomponents into the purifying solution to increase a total fat and oilloading capacity of said solution upon the overshooting of a givenmaximum value or upon a given change in the content of inorganic and/ortotal organic carbon or upon request, or f2) removing fat and oil fromthe solution and reducing the content of inorganic and/or total organiccarbon in the purifying solution upon the overshooting of a givenmaximum value or upon a given change in the content of inorganic and/ortotal organic carbon.
 2. A method as claimed in claim 1, whereinsub-step (d) comprises analyzing inorganic carbon and/or total organiccarbon by converting the carbon of the sample into CO₂ andquantitatively determining the formed CO₂.
 3. A method as claimed inclaim 2, wherein sub-step (d) comprises quantitatively determining theCO₂ absorbed in an absorber solution or a solid absorber and measuringat least one of the following variables: change in electricalconductivity, residual alkalinity, increase in weight.
 4. A method asclaimed in claim 2, wherein sub-step (d) comprises quantitativelydetermining the CO₂ by measuring the infrared absorption.
 5. A method asclaimed in claim 4, comprising measuring the infrared absorption at awavelength of 4.26 μm corresponding to a wave number of 2349 cm⁻¹.
 6. Amethod as claimed in claim 4, wherein a non-dispersive photometer isused to measure the infrared absorption.
 7. A method as claimed in claim1, wherein sub-step (d) comprises extracting lipophilic substances fromsaid solution into an organic solvent not miscible in all proportionswith water and gravimetrically analyzing said lipophilic substances byvaporising off the solvent or photometrically analyzing by infraredabsorption of the lipophilic substances in the extract.
 8. A method asclaimed in claim 7, wherein the infrared absorption of the lipophilicsubstances in the extract is measured at a characteristic oscillatingfrequency of the CH₂ group.
 9. A method as claimed in claim 1,comprising repeating sub-steps (a) to (e) after a predetermined timeinterval.
 10. A method as claimed in claim 1, comprising repeatingsub-steps (a) to (e) after time intervals which are the shorter, thegreater the difference between the results of two consecutive analyses.11. A method as claimed in claim 1, comprising performing sub-steps (a)to (e) in response to an external request.
 12. A method as claimed inclaim 1, wherein in order to determine the content of total organiccarbon, prior to sub-step (d) inorganic carbon is removed from thesample by acidifying the sample and expelling the formed CO₂ with a gas.13. A method as claimed in claim 1, wherein prior to sub-step (d)volatile organic compounds are removed from the sample by expulsion witha gas.
 14. A method as claimed in claim 1, wherein after a predeterminedtime interval or after a predetermined number of analyses or in responseto an external request, the functioning capability of an analyzingdevice used is checked by a control measurement of one or more standardsolutions.
 15. A method as claimed in claim 14, wherein, depending uponthe result of the check of the measuring device used, the analyses ofthe content of inorganic carbon and/or total organic carbon performedbetween tie current and the previous control measurement are providedwith a status characteristic indicating the reliability of theseanalyses of the content of inorganic carbon and/or total organic carbon.16. A method as claimed in claim 14, wherein, depending upon the resultof the check of the measuring device used, the automatic analysis of thecontent of inorganic carbon and/or total organic carbon is continuedand/or one or more of the following actions is performed: analysis ofestablished deviations, correction of the measuring device, terminationof tile analyses of the content of inorganic carbon and/or total organiccarbon, transmission of a status message or alarm signal to asuperordinate process control system or to a monitoring device.
 17. Amethod as claimed in claim 1, wherein the functioning capability of ananalyzing device used is checked by a control measurement of one or morestandard solutions when the results of two consecutive measurementsdiffer by a predetermined amount.
 18. A method as claimed in claim 1,comprising automatically monitoring the levels and/or consumption of thesolutions used and emitting a warning signal upon the undershooting of apredetermined minimum level.
 19. A method as claimed in claim 1,comprising transmitting the results of the analyses and/or of thecontrol measurements and/or of the calibrations and/or the statussignals continuously or at predetermined time intervals and/or uponrequest to a location different to the analysis location.
 20. The methodof claim 1, wherein said step f2) further comprises reducing the fat andoil content of said solution when said measured inorganic and/or totalorganic carbon content of said solution is above a predetermined value.21. The method of claim 1, wherein said method maintains said total fatand oil content of said solution below a predetermined level.
 22. Themethod of claim 1, wherein said method step f1) adds a surfactant tosaid solution to increase the fat loading capacity of said solution whensaid total organic carbon content of said solution is above apredetermined level.
 23. The method of claim 1, wherein said step f2)removes the fat and oil from the solution by a membrane filtrationdevice.
 24. The method of claim 1, wherein said step f1) adds water tosaid solution to dilute said solution and increase said fat and oilloading capacity.